OFDMA and SC-FDMA
In a wireless communication network, such as the 3rd generation (3G) wireless cellular communication standard and the 3GPP long term evolution (LTE) standard, it is desired to concurrently support multiple services and multiple data rates for multiple users in a channel with a fixed bandwidth. The network bandwidth can vary, for example, from 125 MHz to 20 MHz. The network bandwidth is partitioned into a number of subbands, e.g., 1024 subbands for a 10 MHz bandwidth.
One scheme adaptively modulates and encodes symbols, before transmission, based on estimates of a channel. Another option available in LTE, which uses orthogonal frequency division multiplexed access (OFDMA), is to use multi-user frequency diversity by assigning different subbands or groups of subbands to different users or UEs (user equipment, mobile station (MS).
In the single band frequency division multiple access (SC-FDMA) uplink of the LTE, in each UE, the symbols are spread by means of a discrete Fourier transform (DFT) matrix. Then, the symbols are assigned to different subbands.
The following standards are applicable: 36.211, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network: Physical Channels and Modulation (Release 8), v 1.0.0 (2007-March); RI-01057, “Adaptive antenna switching for radio resource allocation in the EUTRA uplink,” Mitsubishi Electric/Nortel/NTT DoCoMO, 3GPP RAN1#48,; R1-071119, “A new DM-RS transmission scheme for antenna selection in E-UTRA uplink,” LGE, 3GPP RAN1#48,; and “Comparison of closed-loop antenna selection with open-loop transmit diversity (antenna switching within a transmit time interval (TTI)),” Mitsubishi Electric, 3GPP RAN1#47bis, Sorrento, Italy. According to the 3GPP standard, the base station (BS) is enhanced, and is called the “Evolved NodeB” (eNodeB). The terms BS and eNodeB are used interchangeably.
Multiple Input Multiple Output (MIMO)
To further increase the capacity of the wireless communication network in fading channel environments, multiple-input-multiple-output (MIMO) antenna technology can be used without an increase in bandwidth. Because the channels for different antennas are different, MIMO decreases fading, and also enables multiple data streams to be transmitted concurrently.
However, processing the signals received in spatial multiplexing schemes, or with space-time trellis codes requires transceivers where the complexity can increase exponentially as a function of the number of antenna.
Antenna Selection
Antennas are relatively simple and cheap, while RF chains are considerably more complex and expensive. Antenna selection reduces some of the complexity drawbacks associated with MIMO networks. Antenna selection reduces the hardware complexity of transmitters and receivers in the transceivers by using fewer RF chains than the number of antennas.
During antenna selection, a subset of the set of available antennas is adaptively selected by a switch, and only signals for the selected subset of antennas are connected to the available RF chains for signal processing, which can be either transmitting or receiving. The selected subset can include one or more of the available antennas.
Pilot Tones or Reference Signals
To select the optimal subset of antennas, channels corresponding to available subsets of antennas need to be estimated, even though only a selected optimal subset of the antennas is eventually used for transmission.
This can be achieved by transmitting antenna selection signals, e.g., pilot tones, also called sounding reference signals (SRS), from different antenna subsets. The different antenna subsets can transmit either the same pilot: tones, or use different pilot tones. Let Nt denote the number of transmit antennas, Nr the number of receive antennas, and let Rt=Nt/Lt and Rr=Nr/Lr be integers. Then, the available transmit (receive) antennas can be partitioned into Rt (Rr ) disjoint subsets.
The pilot repetition approach repeats, for Rt×Rr times, a training sequence that is suitable for an Lt×Lr MIMO network. During each repetition of the training sequence, the transmit RF chains are connected to different subsets of the antennas. Thus, at the end of the Rt×Rr repetitions, the receiver has a complete estimate of all the channels from the various transmit antennas to the various receive antennas.
In case of transmit antenna selection in frequency division duplex (FDD) networks, in which the forward and reverse channels are not identical, the transceiver feeds back the optimal subset of antennas to the transmitter. In reciprocal time division duplex (TDD) networks, the transmitter can perform the selection independently.
For an indoor local area network (LAN) with slowly varying channels, antenna selection can be performed using a media access (MAC) layer protocol, see IEEE 802.11n wireless LAN draft specification, I. P802.11n/D1.0, “Draft amendment to Wireless LAN media access control (MAC) and physical layer (PHY) specifications: Enhancements for higher throughput,” Tech. Rep., March 2006.
Instead of extending the physical (PHY) layer preamble to include the extra training fields (repetitions) for the additional antennas, antenna selection training is done at the MAC layer by issuing commands to the physical layer to transmit and receive packets by different antenna subsets. The training information, which is a single conventional training sequence for an Lt×Lr MIMO network, is embedded in the MAC header field.
SC-FDMA Structure in LTE
The basic uplink transmission scheme is described in 3GPP TR 25.814, v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is a single-band transmission (SC-FDMA) with cyclic prefix (CP) to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver.
Broadband Sounding Reference Signals (SRS)
The broadband SRS helps the eNodeB to estimate the entire frequency domain response of the uplink channel from the UE to the eNodeB. This helps frequency-domain scheduling, in which a subband is assigned to the UE with the best gain on the uplink channel for that subband. Therefore, the broadband SRS can use the entire bandwidth, e.g., 5 MHz or 10 MHz, or a portion thereof as determined by the eNodeB. In the latter case, the broadband SRS is frequency hopped over multiple transmissions to cover the entire network bandwidth.